add an API to output an ed script

[mvdan]

I used them for a project long ago, as a way to easily have portable and readable "edit scripts" to go from plaintext file A to B. You can see it in action via diff --ed.

This is a kind of diff that's unlike the others, since it's not really meant for humans. I still think it belongs here, though.

[mvdan]

I should clarify that this is a "wish list" item. I don't have a need for this right now, nor do I think it's high priority.

[josharian]

From a quick glance, should be pretty straightforward. For a rainy afternoon sometime.